Light emitting diode (LED) lighting has become very popular due to their many advantages including a longer lifespan, fewer hazards, and potentially increased visual appeal when compared to other lighting technologies, such as for example compact fluorescent lamp (CFL) or incandescent lighting technologies. The advantages provided by LED lighting have resulted in LEDs being incorporated into a variety of lighting technologies, televisions, monitors and other applications that may also require dimming.
One known technique for dimming lighting is the use of a triac or phase angle dimming. A triac circuit operates by removing some beginning or ending portion of each half-cycle of ac power, which is known as “leading edge or trailing edge phase control” respectively. By eliminating some portion of each half-cycle, the amount of power delivered to the lamp is reduced and the light output appears dimmed to the human eye. In most applications, the missing portion of each half-cycle is not noticeable to the human eye because the variations in the phase controlled line voltage and the variations of power delivered to the lamp occur so quickly. While the triac dimming circuits work especially well to dim incandescent light bulbs, when they are used for dimming LED lamps they are likely to produce non-ideal results, such as flickering, blinking, color shifting, and input waveform distortions.
A difficulty in using triac dimming circuits with LED lamps comes from a characteristic of the triac itself. A triac behaves as a controlled switch that is open until it receives a trigger signal at a control terminal, which causes the switch to close. The switch remains closed as long as the current through the switch is above certain threshold levels commonly known as a latching current and a holding current. When the triac fires (e.g., turns on) during each half cycle of the input voltage, the current through the switch suddenly increases (typically, in the form of a spike). This spike may lead to ringing in the triac current due to the parasitic capacitances and inductances around the switch, ultimately causing the triac to misfire. Specifically, because of the ringing, the triac may conduct insufficient current to remain engaged and may prematurely turn off. In some cases, even though the current in the triac is compensated (e.g., by having a bleeder circuit drawing additional current from the triac) to remain above the required threshold levels, the rate at which this current drops while ringing may be high enough such that the compensation may not start in time to prevent the triac current from dipping below one or more of those threshold levels.